In recent years there has been growing interest in the use of organic semiconductors, including conjugated polymers, for various electronic applications.
One particular area of importance is the field of organic photovoltaics. Organic semiconductors (OSCs) have found use in OPV as they allow devices to be manufactured by solution-processing techniques such as spin casting and printing. Solution processing can be carried out cheaper and on a larger scale compared to the evaporative techniques used to make inorganic thin film devices. State-of-the-art OPV cells consist of a blend film of a conjugated polymer and a fullerene derivative, which function as an electron donor and an electron acceptor, respectively. In order to achieve highly efficient OPVs, it is important to optimize both the polymer (donor) and fullerene (acceptor) components and to find a material combination yielding an optimal bulk heterojunction (BHJ) morphology that supports efficient exciton harvesting and charge transport properties. Recent improvements in the efficiencies of single-junction OPVs (efficiency ˜8-9%) have largely been due to the development of low-band-gap polymers, which are defined as polymers with an absorption onset of at least 750 nm or more and with a band-gap of 1.65 eV or less. For example, poly(3-hexylthiophene) (P3HT), a low-performance OPV polymer having a band-gap of ˜1.9 eV, is not considered to be a state-of-the-art polymer for OPVs.
The low-band-gap polymer materials and the polymer/fullerene formulations suggested in the prior art for use in OPVs still suffer from certain drawbacks. High-efficiency (>8%) OPVs can be achieved using many different low-band-gap polymers, but all are constrained to use with a specific fullerene, phenyl-C71-butyric-acid-methyl-ester (PC71BM). PC71BM is commercially unacceptable because it is extremely expensive ($325/100 mg) and contains three isomers that are practically impossible to separate. The prior art has indicated that the morphology and performance of the reported high-efficiency, low-band-gap polymers is sensitive to the choice of fullerenes, making the use of PC71BM specifically of critical importance. Replacing PC71BM with another (cheaper) fullerene derivative, such as PC61BM or a non-PCBM fullerene, was previously found to decrease the OPV efficiency from 9.2% to 6-7%.
Though many low band-gap donor polymers have emerged, PC71BM is still the dominant fullerene acceptor, and thus, the progress in OPVs to-date is “one-dimensional” from a materials perspective. The development of polymer/fullerene material systems having morphology which is insensitive to the choice of fullerene significantly increases the degrees of freedom for optimizing polymer/fullerene combinations and exploring many different fullerene derivatives in order to achieve the best OPV performance.
Polymer thieno[3,4-b]thiophene/benzodithiophene (PTB7), described in the prior art, can achieve 9.2% efficiency when combined with the expensive fullerene derivative, PC71BM. The structure of PTB7 is

Replacing PC71BM with a cheaper fullerene PC61BM decreases the OPV efficiency to 6.5%. A slight change in the substitution group of fullerene, such as changing PC71BM to PC71PM, which is only one carbon shorter than PC71BM, causes a dramatic decrease in OPV efficiency to ˜1%. State-of-the-art OPV polymers, such as PTB7, are extremely sensitive to the choice of fullerene used, and therefore high-efficiency OPV can only be achieved with PC71BM.

The aim of the presently claimed subject matter is to provide polymer/fullerene formulations for use as organic semiconducting materials that do not have the drawbacks of the prior art materials as described above, are easy to synthesize, especially by methods suitable for mass production, and show advantageous properties, especially for OPV use, as described above.
Another aim of the presently claimed subject matter is to extend the pool of fullerene materials that can be matched with high-performance, low-band-gap polymers to produce highly efficient OPVs. In this regard, it is important to develop polymer/fullerene material systems whose morphology and performance is insensitive to the choice of fullerenes.